Two-terminal spintronic devices

ABSTRACT

This disclosure describes an example device that includes a first contact line, a second contact line, a spin-orbital coupling channel, and a magnet. The spin-orbital coupling channel is coupled to, and is positioned between, the first contact line and second contact line. The magnet is coupled to the spin-orbital coupling channel and positioned between the first contact line and the second contact line. A resistance of the magnet and spin-orbital coupling channel is a unidirectional magnetoresistance.

The application claims the benefit of U.S. Provisional Application No.62/434,166, filed Dec. 14, 2016, the entire content of which isincorporated by reference herein.

GOVERNMENT INTEREST

This invention was made with government support under Grant No.HR0011-13-3-0002 awarded by Department of Defense/Defense AdvancedResearch Projects Agency (DARPA). The government has certain rights inthe invention.

TECHNICAL FIELD

This disclosure relates to articles including magnetic structures, andmore particularly, magnetic structures for memory and logic devices.

BACKGROUND

The scaling of conventional semiconductor devices may be limited byfactors including device reliability and increased power consumption.Improvement in the performance of memory and computational devices iscontinuously pursued. Spin-based or spintronic devices may be used asalternatives to or in conjunction with electronic devices. Spin-basedeffects may be used by devices such as spintronic devices that harnessthe intrinsic spin of electrons and their associated magnetic moments,in addition to electronic phenomena that arise from the fundamentalelectronic charges of electrons. Magnetic structures may be used inspintronic devices including memory and computational devices. Forexample, memory devices such as magnetic random access memory (MRAM) orspin-transfer torque random access memory (STT-RAM) may be based on therelative magnetic orientation of multiple magnetic layers.

SUMMARY

In general, the disclosure describes examples of two-terminal spintronicdevices such as memory or logic devices. The two-terminal spintronicdevices may be based on a unidirectional resistance of a nonmagneticmaterial (e.g., a spin-orbital coupling channel) and a ferromagneticmaterial (e.g., a magnet), which may be referred to as unidirectionalspin Hall magnetoresistance (USMR) effects. For USMR effect, current ofa threshold amplitude through a non-magnetic (NM) material that hasstrong spin-orbit interaction (SOI) causes a magnetization direction ofa ferromagnetic (FM) layer to switch. The magnetization direction of theFM layer may be correlated with the resistance at the interface betweenthe FM material and the NM material. In some examples, the resistance ofthe FM material and the NM material may be constant. In other words, theresistance of the FM material alone and the resistance of the NMmaterial alone may not change, but the total resistance of thecombination of the FM material and the NM material may be aunidirectional magnetoresistance that changes depending on themagnetization direction of the FM layer (e.g., the resistance at theinterface between the FM material and the NM material may change, whichmay cause a change in the total resistance). By applying a currentgreater than the threshold current density (e.g., amplitude, but notlimited to amplitude) through the NM material, a controller circuit mayset the resistance of the combined NM material and FM material (e.g.,the current may change the resistance at the interface). Then, byapplying a current less than the threshold current density through theNM material, the controller circuit may read a voltage across thecombination of the NM material, the FM material, and the interfacebetween the NM material and the FM material, such that the controllercircuit may determine the resistance of the combination of the NMmaterial, the FM material, and the interface. In this way, theresistance of the device may be used as way to convey a digital high ora digital low.

In some examples, the disclosure describes a device that includes afirst contact line, a second contact line, a spin-orbital couplingchannel, and a magnet. The spin-orbital coupling channel is coupled to,and is positioned between, the first contact line and second contactline. The magnet is coupled to the spin-orbital coupling channel andpositioned between the first contact line and the second contact line. Aresistance of the magnet and spin-orbital coupling channel is aunidirectional magnetoresistance. The magnet is directly coupled to thespin-orbital coupling channel and at least one of the first contact lineor the second contact line. The spin-orbital coupling channel isdirectly coupled to the at least one of the first contact line or thesecond contact line.

In another example, the disclosure describes a device that includes afirst contact line, a second contact line, a third contact line, a firstspin-orbital coupling channel, a second spin-orbital coupling channel, afirst magnet, and a second magnet. The first spin-orbital couplingchannel is positioned between the first contact line and second contactline and directly coupled to at least one of the first contact line orthe second contact line. The second spin-orbital coupling channel ispositioned between the second contact line and the third contact lineand directly coupled to at least one of the second contact line or thethird contact line. The first magnet is positioned between the firstcontact line and the second contact line and directly coupled to thefirst spin-orbital coupling channel. The second magnet is positionedbetween the second contact line and the third contact line and directlycoupled to the second spin-orbital coupling channel. A resistance of thefirst magnet and the first spin-orbital coupling channel is aunidirectional magnetoresistance and a resistance of the second magnetand second spin-orbital coupling channel is a unidirectionalmagnetoresistance.

In yet another example, the disclosure describes a device that includesa first contact line, a second contact line, a spin-orbital couplingchannel, a magnet, and a controller circuit. The spin-orbital couplingchannel is configured to receive a read current and a write current. Thespin-orbital coupling channel is coupled to, and is positioned between,the first contact line and second contact line. The magnet is coupled tothe spin-orbital coupling channel and positioned between the firstcontact line and the second contact line. A resistance of the magnet andspin-orbital coupling channel is a unidirectional magnetoresistance. Thecontroller circuit is configured to: output a write current through thespin-orbital coupling channel to set a resistance of the magnet to afirst resistance level indicative of a first digital value or a secondresistance level indicative of a second digital value; and output theread current through the spin-orbital coupling channel to determinewhether the resistance is at the first resistance level or the secondresistance level, without outputting a current through the magnet.

The details of one or more embodiments of the invention are set forth inthe accompanying drawings and the description below. Other features,objects, and advantages of the invention will be apparent from thedescription and drawings, and from the claims.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a conceptual diagram illustrating an example of spin Halleffect.

FIG. 2A is a block diagram illustrating an example of a three-terminalspintronic device.

FIG. 2B is a graph illustrating a change in resistance of the device ofFIG. 2A as a function of current.

FIGS. 3A and 3B are conceptual diagrams illustrating unidirectional spinHall magnetoresistance (USMR) effects.

FIGS. 4A and 4B are conceptual diagrams illustrating writing to andreading from, respectively, a two-terminal spintronic device.

FIGS. 5A-5C are conceptual diagrams illustrating examples of crossbarand 3D architectures for memory devices using two-terminal spintronicdevices.

FIGS. 6A-6D are conceptual diagrams illustrating examples of memory celllayouts.

FIGS. 7A and 7B are conceptual diagrams illustrating examples of crossbar memory architecture.

FIG. 8A is a conceptual diagram illustrating an example of nanowirecell.

FIG. 8B is a conceptual diagram illustrating an example of a nanowirecell in cross bar.

FIGS. 9A an 9B are conceptual diagrams illustrating an example forsensing magnetic nano-particles.

FIG. 9C is a graph illustrating example threshold for sensing presenceof magnetic nano-particles.

FIG. 10A is a conceptual diagram illustrating resistance measurementsetup and definitions of rotation planes.

FIGS. 10B and 10C are graphs illustrating resistance as a function ofangle.

FIG. 11A is a conceptual diagram illustrating transverse/Hall resistancemeasurement setup.

FIGS. 11B and 11C are graphs illustrating resistance as a function ofangle and external fields, respectively.

FIGS. 12A and 12B are graphs illustrating contribution for totalresistance for different examples of structures that exhibit USMReffects.

FIGS. 13A and 13B are graphs illustrating USMR per current density pertotal resistance and sheet USMR per current density, respectively, as afunction of temperature.

FIG. 14 is a graph illustrating resistance as a function of magneticfield for different structures.

FIGS. 15A-15C are graphs illustrating resistance as a function ofmagnetic field for additional structures are different temperatures.

FIG. 16 is a graph illustrating sheet USMR comparison.

FIG. 17 is a flowchart illustrating example operations of a deviceconfigured to write to and read from a two-terminal spintronic device.

DETAILED DESCRIPTION

This disclosure describes a two-terminal spintronic device, which mayovercome limitations of three-terminal spintronic devices for memory andcomputation applications. The example techniques may be applicable tosupport a magnetic cross-bar memory architecture, a magnetic 3D memoryarchitecture, and computation in magnetic memory architecture.

Current semiconductor devices face many challenges and bottlenecksincluding the difficulty to further scale, increased dynamic and staticpower consumption, and limitations of speed. Instead of using the chargeof electrons to represent, store, transfer and compute information, thespin momentum of electrons can also be used. The technology of utilizingelectron spins is called Spintronics.

Spintronics may feature better scalability, better speed, less powerconsumption, and non-volatility. Example uses of spintronics such as inspin transfer torque random access memory (STT-RAM) have alreadyemerged. However, devices like STT-RAM may still exhibit issues such aslower switching efficiency, which in turn leads to worse powerconsumption and worse reliability. The spin Hall effect in some heavymetals and the topological insulators may be alternatives to achievemagnetization switching more efficiently than STT.

The spin Hall effect (SHE) is a phenomenon of electron spins to deflecttransversely when a charge current is applied in a non-magnetic (NM)material that has strong spin-orbit interaction (SOI). This may lead topolarized spins flow, which is called spin current, and spinaccumulation at interfaces. If a ferromagnetic (FM) layer is in contactwith the NM layer, the spins generated by the SHE will interact with themagnetization of the FM due to the transfer of angular momentum from thespin to the magnetization. In some cases, with the transfer of theangular momentum, the magnetization in FM can be switched by utilizingthe SHE.

Topological insulators (TIs) are a kind of material whose bulk iselectrically insulating but whose surfaces may be conductive. Theelectrons flowing on the surface of a TI may be spin polarized (e.g.,completely spin polarized) due to the spin-momentum locking of thesurface states. These electron spins may exert large torques onmagnetization nearby. Although TIs possess different physics compared toheavy metals, TIs may generate spins and switch magnetizations in asimilar way in terms of the device structure, directions of current andtorque. Accordingly, both heavy metals and TIs may be considered asfunctioning in a SHE or SHE-like fashion when used to switch a magnet.

However, while such devices can be made more efficient to switch usingthe SHE or TIs, the device may require a third terminal and a magnetictunneling junction (MTJ) structure in order to read out themagnetization state. This makes such devices more difficult to fabricateand weakens their scalability. Stated another way, SHE or TIs may beused to set the magnetization state of a magnet to a first magnetizationstate or a second magnetization state. The first magnetization statecorresponds to a first digital value (e.g., logic high or low), and thesecond magnetization state corresponds to a second digital value (e.g.,other of the logic high or low). As described above, the magnetizationstate is set by driving a current through the NM material. In this case,the input of the NM material and the output of the NM material formfirst and second terminals, respectively. In this way, SHE or TIs areused to store a digital value on the magnet.

However, reading the digital value, by determining the magnetizationstate of the magnet, may require another terminal and an MTJ structurethat includes a free FM layer and a fixed FM layer. Conventionally, theresistance of a single magnetic layer in a first magnetization state isthe same as the resistance of the single magnetic layer in a secondmagnetization state. Thus, in order to store and subsequently read thedigital value, a conventional memory or logic device that utilizes theSHE includes a MTJ structure with a free FM layer and a fixed FM layerbecause the resistance of the MTJ as a whole changes depending onwhether the free FM layer is parallel or anti-parallel to the fixed FMlayer. Thus, when the device includes an MTJ, a current is outputthrough the MTJ (which forms the third terminal) to determine theresistance of the MTJ and hence the digital value stored by the memorydevice.

This disclosure describes examples where a two-terminal spintronicdevice may be used, rather than the three-terminal example describedabove. The use of such two-terminal devices reduces needed components,which allows for the spintronic devices to be scaled down even further.Such reduction in size and reduction in terminals may be beneficial forexamples of memories and logic devices.

For example, devices that exhibit a unidirectional resistance of aspin-orbital coupling channel (e.g., a spin hall channel) and a magnet,which may be referred to as unidirectional spin Hall magnetoresistance(USMR), may provide for devices with two terminals that do not requirean MTJ for reading the magnetization state of the magnet. In someexamples, the unidirectional magnetoresistance of the spin-orbitalcoupling channel may change from a first resistance value to a secondresistance value depending on the magnetization direction of the magnet(e.g., the magnetization direction of the magnet may affect theresistance of an interface between the magnet and the spin-orbitalcoupling channel).

According to techniques of this disclosure, unidirectional resistancemeans the resistance of the spin-orbital channel and the magnet is afirst resistance level when the magnetization direction of the magnet isa first direction, and the resistance is a second resistance level whenthe magnetization direction is a second direction. For instance, theunidirectional magnetoresistance may be a first value when themagnetization direction of the magnet is at approximately 0 degrees(e.g., relative to a reference direction, which may be defined by thedirection of the electron spin of the spin-orbital coupling channel) anda second value when the magnetization direction of the magnet isapproximately 180 degrees. In some examples, the resistance values maybe different at magnetization directions other than 0 degrees and 180degrees depending on the structure of the magnet. For example, the firstmagnetization direction may be approximately 45 degrees (e.g., ±44.99degrees) and the second magnetization direction may be approximately 135degrees (e.g., ±44.99 degrees). As another example, the firstmagnetization direction may be approximately 0° (e.g., ±up to 89.99°)and the second magnetization direction may be approximately 180° (e.g.,±up to 89.99°). In other examples, the first magnetization direction maybe approximately 0° (e.g., ±45°) and the second magnetization directionmay be approximately 180° (e.g., ±45°). As yet another example, thefirst magnetization direction may be approximately 30° (e.g., ±59.99°)and the second magnetization direction may be approximately 150° (e.g.,±59.99°). In some examples, the magnetization directions may be definedin the 180° to 360° range.

In some examples, the resistance may be different for more than twomagnetization states. In other words, the resistance may be a firstvalue when the magnetization direction is approximately 0 degrees, asecond value when the magnetization direction is approximately 90degrees, and a third value when the magnetization direction isapproximately 180 degrees.

In a simple FM/NM structure, due to the spins accumulation from SHE atthe interface, the USMR is present. The unique angular dependency ofUSMR, which is also left-right sensitive like the tunnelingmagnetoresistance, makes it a substitute for three terminal devices.Thus, with USMR, a simple device made out of NM/FM bilayer may becapable of both writing and reading without any additional (third)terminal or MTJ structure. USMR may also be present in TI/FM systems,thus making the example techniques applicable in the same way for TI/FMdevices.

This disclosure describes examples of combining the spin-orbit torqueswitching by SHE or TIs as writing mechanism with USMR as a readingmechanism for a simple, yet potentially powerful design of amemory/logic device featuring only two terminals. This allows the use ofmore efficient spin-orbit torque (SOT) switching while still keeping thedevice at minimal two terminals so that it can be easily embedded intomature crossbar memory architectures, with or without selectors, whichare used, for example, with MTJs in STT-RAM.

As described above, the unidirectional resistance refers to the combinedresistance of the magnet and the spin-orbital coupling channel. Themagnetization direction of the magnet sets the resistance of thecombination of the magnet and the spin-orbital coupling channel (e.g.,by setting the interface of magnet and the spin-orbital couplingchannel), and therefore, the resistance is not based on themagnetization direction of the magnet relative to a magnetizationdirection of another layer (sometimes called fixed layer for MTJs).

FIG. 1 is a conceptual diagram illustrating an example of spin Halleffect. In the illustrated example, a charge current (Jc) is passedthrough a spin-orbital coupling channel 102. In some examples,spin-orbital coupling channel 102 may be a spin Hall channel, a spinchannel that provides a Rashba effect with a magnet, or a channel thatprovides a unidirectional interface effect to induce the USMR effect.Due to the spin orbit interactions (SOI), electrons 114 of differentspin directions are deflected in directions that are at right angles totheir spins. As illustrated in FIG. 1, electrons deflected up/down carryspins pointing to right/left, respectively. At the surface/interface ofthe spin-orbital coupling channel 102, these spin-polarized electronsaccumulate, which is referred to as spin accumulation. The accumulatedspins of electrons 102 can potentially exert a torque on magnet 104 ifthe magnet 104 is in close contact to a surface of the spin-orbitalcoupling channel 102. The torque may switch the magnetization state(e.g., magnetization direction) of the magnet 104.

FIG. 2A is a block diagram illustrating an example of a three-terminalspintronic device. FIG. 2A illustrates an MTJ 204 that includes a CoFeBfixed layer, a MgO insulation layer, and a CoFeB free layer. Themagnetization direction (e.g., magnetization state) of the MTJ 204, andparticularly the CoFeB free layer, changes based on the flow of thecurrent through the spin-orbital coupling channel 202. As illustrated,spin-orbital coupling channel 202 is formed by Ta. Other examplematerials for MTJ 204 and spin-orbital coupling channel 202 arepossible. Spin Hall effect can completely switch the magnetization ofthe free layer of an in-plane magnetic layer (e.g., CoFeB free layer ofMTJ 204).

FIG. 2B is a graph illustrating a change in resistance of the device ofFIG. 2A as a function of current. The resistance of MTJ 204 of FIG. 2Amay be based on the whether the magnetization of the free layer and thefixed layer are in the same direction (referred to as parallel state)for low impedance, or in opposite direction (referred to asanti-parallel) for high impedance. FIG. 2B illustrates that is possibleto change the magnetization direction of the free layer based on thecurrent flowing through the spin-orbital coupling channel 202.

MTJ 204 may be used for purposes of writing a digital value that issubsequently read. In this way, MTJ 204 may function as part of a memorycell. For instance, the resistance of the MTJ 204 may be associated witha digital value. When the MTJ 204 has a high resistance, then the MTJ204 may be associated with a first digital value (e.g., digital high ordigital low), and when the MTJ 204 has a low resistance, then the MTJ204 may be associated with a second digital value (e.g., the other ofthe digital high or digital low).

A controller circuit (not illustrated) may output a current through thespin-orbital coupling channel 202 that sets the MTJ 204 into theparallel state or anti-parallel state. For instance, the current mayflow through terminals 208 and 210. When MTJ 204 is in the parallelstate, the MTJ 204 may be considered as storing the first digital value,and when MTJ 204 is in the anti-parallel state, the MTJ 204 may beconsidered as storing the second digital value. In this example, a firstterminal 208 may be the input into the spin-orbital coupling channel202, and a second terminal 210 may be the output of the spin-orbitalcoupling channel 202. By setting the MTJ 204 to parallel oranti-parallel, the controller circuit may effectively write a first or asecond digital value.

To read the digital value, the controller circuit may output a currentthrough the MTJ 204 via terminal 212 and measure the voltage. If the MTJ204 is in the parallel state, then the voltage will be at a firstvoltage value because the resistance of the MTJ 204 will be low, and ifthe MTJ 204 is in the anti-parallel state, then the voltage will be asecond voltage value because the resistance of the MTJ 204 will be high.Accordingly, by measuring the voltage, the controller circuit maydetermine the resistance of the MTJ 204, and hence, the digital valuestored by the MTJ 204. In this example, the terminal through which thecurrent flows through the MTJ 204 is a third terminal 212, meaning thatthe example illustrated in FIG. 2A is for a three-terminal device.

Accordingly, in the example illustrated in FIG. 2A, three terminals maybe needed for writing and reading. This disclosure describes exampletechniques to reduce from three terminals for writing and reading, tousing two-terminals for writing and reading.

While spin Hall effects may have led to many memory and logic deviceconcepts and demonstrations, the devices may be for three-terminaldevices. Developing three-terminal devices, including the MTJ structuremay be complicated and may not suitable for cross-bar memory and 3Dmemory architecture. For example, forming such stacks of MTJ structureson top of one another or in a cross-bar architecture may be complicatedand may not result in consistent structures exhibiting the sameproperties.

This disclosure describes examples of two-terminal spintronic devices.To achieve two-terminal spintronic devices, the disclosure describesusing unidirectional spin Hall magnetoresistance (USMR) effects. InUSMR, the resistance of a spin-orbital coupling channel, a magnet, andthe interface between the channel and the magnet, is set based on thecurrent through a spin-orbital coupling channel. However, no additionalterminal is needed to determine the resistance of the magnet. Rather, acurrent through the spin-orbital coupling channel can be used todetermine the voltage across the channel, magnet, and interface, andhence the resistance of the channel, magnet, and interface.

FIGS. 3A and 3B are conceptual diagrams illustrating unidirectional spinHall magnetoresistance (USMR) effects. Spins are generated at aninterface of a non-magnetic layer 302 (e.g., a topological insulator(TI)) and a magnetic layer 304 (e.g., CoFeB) when a charge current (j)is applied to the non-magnetic layer 302. The relative directions ofspins to magnetization of either parallel or anti-parallel result indifferent resistance states or levels. For example, as illustrated inFIG. 3A, the spin of electrons 314 is anti-parallel to the magnetizationdirection of magnetic layer 304, which may in some examples correspondto a high resistance level. In the example of FIG. 3B, the spin ofelectrons 314 is parallel to the magnetization direction of magneticlayer 304, which may correspond to a low resistance level. A highresistance level (e.g., greater than or equal to a threshold resistance)may correspond to a first digital value (e.g., one of a digital high ora digital low), while a low resistance level (e.g., less than athreshold resistance) may correspond to a second digital value (e.g.,the other one of a digital high or a digital low). In contrast toconventional three-terminal devices that utilize an MTJ where theresistance is based on the magnetization direction of a free magnetrelative to the magnetization direction of a fixed magnet (e.g., thefixed magnet acts as a reference), according to techniques of thisdisclosure, the resistance is based on the magnetization direction of asingle magnet relative to the spin direction of electrons in thespin-orbital coupling channel. In other words, the unidirectionalresistance is not based on the magnetization direction of the magnetrelative to a magnetization direction of another magnet. Said yetanother way, the reference direction may be the direction of theelectron spins of the spin-orbital coupling channel and not thedirection of a reference magnet.

The USMR originates from the spin accumulation at the FM(ferromagnetic)/NM (non-magnetic) interface induced by spin Hall effect.It is unique compared to anisotropic magnetoresistance or spin Hallmagnetoresistance due to its angular symmetry. It may be one of few(possibly only) type of magnetoresistance in a FM/NM structure thatgives sensitivity to magnetization states that are opposite to eachother. For example, tunneling magnetoresistance (e.g., such as in anMTJ) may require an additional reference FM.

FIGS. 4A and 4B are conceptual diagrams illustrating an example system400 for writing to and reading from, respectively, a two-terminalspintronic device. For instance, FIGS. 4A and 4B illustrate thecombination of spin Hall effect induced switching and USMR fortwo-terminal memory device. System 400 includes two-terminal memorydevice 401 and controller circuit 420.

In some examples, two-terminal memory device 401 includes contacts 422Aand 422B (collectively, contacts 422), and structure 410. Structure 410includes spin-orbital coupling channel (e.g., a spin hall channel) 402and magnet 404. Contacts 422A and 422B may include any conductivematerial and may be configured to transport a voltage or current fromcontroller circuit 420 through spin-orbital coupling channel 402. Inother words, contacts 422A and 422B may function as terminals fortwo-terminal memory device 401.

Spin-orbital coupling channel 402 may include non-magnetic material,such a heavy metal (e.g., Tungsten (W), Tantalum (Ti), Platinum (Pt)) ortheir alloys or their multilayers, a topological insulator (TI), a dopedtopological insulator, or a combination therein. In some examples, atopological insulator may include Bi₂Se₃ or (BiSb)₂Te₃.

Magnet 404 may include a ferromagnetic material (e.g., Fe, CoFeB). Themagnet 404 may not need to be tailored differently from spin-orbitalcoupling channel 402 (e.g., magnet 404 and spin-orbital coupling channel402 may be the same shape or different shapes) as the spin-orbitalcoupling channel 402 since the effects may be based on the interface403, or boundary, between spin-orbital coupling channel 404 and magnet404. For example, a spin hall current along the surface of spin-orbitalcoupling channel 402 may exert a spin torque along the interface ofspin-orbital coupling channel 402 and magnet 404, which may change themagnetization state of magnet 410, and hence the resistance of structure410. For instance, changing the magnetization of magnet 404 may changethe resistance at the interface 403 between magnet 404 and spin-orbitalcoupling channel 402, such that the resistance of structure 410 changes.

In accordance with techniques of this disclosure, controller circuit 420may control write operations to, and read operations from, two-terminalmemory device 401. Controller circuit 420 may include one or moreprocessors, including, one or more microprocessors, digital signalprocessors (DSPs), application specific integrated circuits (ASICs),field programmable gate arrays (FPGAs), or any other equivalentintegrated or discrete logic circuitry, as well as any combinations ofsuch components.

As illustrated in FIG. 4A, the initial magnetization direction of magnet404 may be illustrated by a dashed arrow 406A. Different magnetizationdirections of magnet 404 may correspond to different resistance levels(e.g., a high resistance level or a low resistance level) of structure410. The resistance level of structure 410 may be considered to be ahigh resistance level when the resistance satisfies (e.g., is greaterthan or equal to) a first threshold resistance and may be considered tobe a low resistance level when the resistance does not satisfy (e.g., isless than) a second threshold resistance. The first threshold resistanceand the second threshold resistance may be the same resistance value, ormay be different resistance values. Each resistance level may correspondto a different digital value, such as a digital high (e.g., a “1”) or adigital low (e.g., “0”). In other words, the resistance value mayrepresent, or be indicative of, a digital value. In some examples, ahigh resistance level corresponds to a digital high and a low resistancelevel corresponds to digital low. In some examples, a high resistancelevel corresponds to a digital low and a low resistance levelcorresponds to digital high.

To write, controller circuit 420 may apply a strong pulse (e.g., greaterthan or equal to a threshold current density) across the two-terminalmemory device 401 so that spins of electrons 414 are generated by spinHall effect in the channel 402 and are absorbed by the top magnet 404.In other words, controller circuit 420 may apply a write current tospin-orbital coupling channel 402 by applying a strong pulse totwo-terminal memory device 401. In response to controller circuit 420applying the write current across two-terminal device 401, themagnetization (illustrated by the bolded white arrow 406B) then isswitched from left to right (block lines illustrate an example of onepossible trajectory 407). In some examples, a resistance level ofspin-orbital coupling channel 402 and a resistance level of magnet 404may be constant, while a resistance level of an interface 403 betweenchannel 402 and magnet 404 may vary based on the magnetization directionof magnet 404 (e.g., the resistance of interface 403 may beunidirectional). The resistance level of structure 410 includes theresistance of channel 402, the resistance of magnet 404, and theresistance of interface 403. In some examples, controller circuit 420changes the overall resistance level of structure 410 by changing themagnetization direction of magnet 404 (e.g., which may change theresistance at the interface 403 between magnet 404 and spin-orbitalcoupling channel 402), which may change the overall resistance level ofstructure 410, and hence change the digital value represented by themagnetization state of magnet 404. Because the resistance level ofstructure 410 is unidirectional and is based on the magnetizationdirection of magnet 404, the resistance level structure 410 may be afirst resistance level when a magnetization direction of the magnet 404is a first direction (e.g., 0 degrees) and a second resistance levelwhen the magnetization direction of the magnet 404 is 180 degreesopposite the first direction (e.g., 180 degrees).

For instance, when the initial magnetization direction of magnet 404causes the resistance level of structure 410 to be a first resistancelevel (e.g., greater than a threshold resistance), such that themagnetization state may represent a first digital value (e.g., digitalhigh). Responsive to receiving a write current, the magnetizationdirection of magnet 404 may change, which may cause the resistance ofstructure 410 to change (e.g., by changing the resistance of theinterface 403) from a first resistance level to a second resistancelevel, such that the magnetization state may represent a second digitalvalue (e.g., digital low).

To read, controller circuit 420 may apply a mild current (e.g., lessthan the threshold current density), and the controller circuit 420 maydetermine the voltage signal across the structure 410. In other words,controller circuit 420 may apply a read current to spin-orbital couplingchannel 402 to determine the voltage across structure 410 (e.g., thevoltage across spin-orbital coupling channel 402, magnet 404, and theinterface between channel 402 and magnet 404). The mild current may besine wave modulation. The measured voltage, and hence resistance,indicates the state of the magnet 404. Said differently, by measuringthe voltage across structure 410, controller circuit 420 may determinethe resistance level of structure 410, and hence the digital valuerepresented by magnet 404. Applying the read current to channel 402 mayenable controller circuit 420 to determine the magnetization state ofmagnet 404 without utilizing a third terminal, such that two-terminalspintronic device 401 may function as a memory or logic device with onlytwo terminals.

FIGS. 5A-5C are conceptual diagrams illustrating examples of crossbarand 3D architectures for memory or logic devices using two-terminalspintronic devices. With the two terminals, the example devicesdescribed in this disclosure may be embedded into crossbar architectures(FIGS. 5A and 5B) or 3D architectures (FIG. 5C). In accordance with someexamples, two-terminal devices without a selector may be added withoutadding the third set of contact lines to support read operations (e.g.,in contrast to devices with an MTJ without a selector which may usethree sets of contact lines or terminals). As another example, twoterminal devices with a selector may be added without adding a fourthset of lines to support read operations (e.g., in contrast to deviceswith an MTJ and selector which may use four sets of lines or terminals).In some examples, a selector includes a switch (e.g., a transistor)connected to the controller via a contact line or terminal and enablesthe controller to selectively read the memory or logic device byallowing current to flow through the memory or logic device when theselector is activated (e.g., the switch on) and preventing current fromflowing through the memory or logic device when the selector is inactive(e.g., the switch is off).

In other words, according to techniques of this disclosure, the memoryor logic devices may include one less terminal compared to devices thatutilize MTJs. The two-terminal devices may provide better compatibilitywith the current peripheral circuitries found in STT-RAM for write andread operations, and with higher density as compared to three-terminaldesigns. Also, given proper thin film growth processes, the device maybe made vertically, which allows for even higher areal density.

Electric field and strain effects may also be used to further assistwith switching with the techniques described in this disclosure. Thetechniques may keep the device to minimal two terminals which makes iteasy to adopt with the current STT-RAM technologies. The device can bemade with a bilayer sandwich structure to further ease the fabricationeffort.

Accordingly, this disclosure describes examples of a device (e.g.,memory or logic device) with two-terminal write and read operations thatacts like an MTJ but carries greater switching efficiency as spin Halleffect is utilized. The two-terminal device may support magneticcross-bar and 3D memory architectures.

FIGS. 5A, 5B, and 5C illustrate example memory (or logic) devices 501A,501B, and 501C, respectively (collectively, memory devices 501). Memorydevices 501A and 501B are illustrated in a crossbar architecture. Eachdevice of memory devices 501A and 501B includes a first contactor (alsoreferred to as a contact line or terminal) 522A, a second contactor522C, and a structure 510A that includes spin-orbital coupling channel502A and magnet 504A. In some examples, as illustrated in FIG. 5A,memory device 501A may include a third contactor.

Spin-orbital coupling channel 502 and magnet 504 may be alignedhorizontally (e.g., as illustrated in FIG. 5A) or vertically (e.g., asillustrated in FIG. 5B). In some examples, when spin-orbital couplingchannel 502A and magnet 504A are aligned horizontally, an interface 503Abetween spin-orbital coupling channel 502A and magnet 504A may define aplane that is substantially parallel to a plane defined by the interfacebetween magnet 504A and first contactor 522A. As another example, whenspin-orbital coupling channel 502A and magnet 504A are alignedvertically, an interface 503A between spin-orbital coupling channel 502Aand magnet 504A may define a first plane that is substantiallyperpendicular to a second plane defined by the interface between magnet504A and first contactor 522AA, a third plane defined by the interfacebetween spin-orbital channel 502A and first contactor 522A, or both. Insome examples, the second plane and the third plane are substantiallyplanar with one another.

In some examples, magnet 504A may be not be directly coupled to any ofthe contactors or may be directly coupled to a single contactor. Forexample, as illustrated in FIG. 5A, magnet 504A may be coupled to thecontactors indirectly (e.g., via magnet 504A). In some examples,spin-orbital coupling channel 502A and magnet 504A are each directlycoupled to at least one contactor (e.g., first contactor 522A, secondcontactor 522B, or both). For example, as illustrated in FIG. 5A,spin-orbital coupling channel 502A may be directly coupled to twocontactors (e.g., first contactor 522A and third contactor 522C), and asillustrated in FIG. 5B, spin-orbital coupling channel 502A and magnet504A may each be directly coupled to first contactor 522A and secondcontactor 522B. In some examples, the spin-orbital coupling connector isdirectly coupled to one contactor (e.g., as illustrated in FIG. 6B).

FIG. 5C illustrates an example memory device 501C in a 3D architecture.In some examples, a memory or logic device may include a plurality ofspin-orbital coupling channels and magnets. For instance, device 501Cincludes spin-orbital coupling channels 502A-502C (collectively,spin-orbital coupling channels 502) and magnets 504A-504C (collectively,magnets 504). Structure 510A includes spin-orbital coupling channel 502Aand magnet 504A, structure 510B includes spin-orbital coupling channel502B and magnet 504B, and structure 510C includes spin-orbital couplingchannel 502C and magnet 504C. In the examples of FIGS. 5A-5C, a topcontactor (e.g., contactor 522A) and a bottom contactor (e.g., contactor522B) may sandwich a spin-orbital coupling channel (e.g., spin-orbitalcoupling channel 502A) and a magnet (e.g., magnet 502A). In other words,spin-orbital coupling channel 502A and magnet 504A may be locatedbetween two contactors. In some instance, a third contact line or via(e.g., contactor 522C) and/or a selector (e.g., as illustrated in FIGS.6A-6D) may also be located between the two contactors. As illustrated inFIG. 5C, spin-orbital coupling channel 502A and magnet 504A are alignedhorizontally such that an interface 503A between spin-orbital couplingchannel 502A and magnet 504A may define a plane that is substantiallyparallel to a plane defined by the interface between magnet 504A andfirst contactor 522A. Similarly, interfaces 503B and 503C may alsodefine planes that are substantially parallel to planes defined betweenthe respective magnets and contactors. However, in some examples, suchas illustrated in FIG. 7B, a spin-orbital coupling channel and magnetmay be aligned vertically, such that an interface between thespin-orbital coupling channel and magnet defines a plane that issubstantially perpendicular to a plane defined by the interface betweenthe magnet and first contactor and the interface between thespin-orbital channel and the first contactor.

As one example, the disclosure describes a device that includes aspin-orbital coupling channel (e.g., 502A) configured to receive a writecurrent and a read current, a magnet (e.g., 504A) coupled to thespin-orbital coupling channel, wherein the magnet and spin-orbitalcoupling channel exhibit unidirectional spin Hall magnetoresistance(USMR) effects, and a controller circuit (e.g., controller circuit 420illustrated in FIGS. 4A-4B). The controller circuit 420 may beconfigured to output the write current through the spin-orbital couplingchannel 502A to set a magnetization direction of the magnet 504A, whichmay set the resistance of structure 510 (e.g., by changing theresistance of interface 503A) to a first resistance level indicative ofa first digital value or a second resistance level indicative of asecond digital value, and output the read current through thespin-orbital coupling channel 502A to determine whether the resistanceof structure 510 is at the first resistance level or the secondresistance level, without outputting a current through the magnet 504A.

In some examples, the controller circuit is configured to determine avoltage across structure 510A that includes spin-orbital couplingchannel 502A and the magnet 504A, and determine whether the resistanceof the structure 510A is at the first resistance level or the secondresistance level based on the voltage across the structure 510A. Thewrite current may be at a first current density greater than or equal toa threshold current density for setting a magnetization direction of themagnet, and the read current may be at a second current density lessthan the threshold current density. The magnetization direction includesone of a first magnetization direction for which the resistance of thestructure 510 is the first resistance level, or a second magnetizationdirection for which the resistance of the structure 510 is the secondresistance level. The spin-orbital coupling channel may be formed from atopological insulator or a heavy metal in some examples.

The spin-orbital coupling channel 502A may be a first spin-orbitalcoupling channel, the magnet 504A may be a first magnet, the writecurrent may be a first write current, and the read current may be afirst read current. In some examples, such as the example illustrated inFIG. 5C, the device may also include at least a second spin-orbitalcoupling channel 502B configured to receive a second write current and asecond read current, and at least a second a magnet 504B coupled to thesecond spin-orbital coupling channel 502B, wherein the second magnet andsecond spin-orbital coupling channel exhibit the USMR effects (e.g.,unidirectional resistance at the interface 503B). The controller may beconfigured to output the second write current through the secondspin-orbital coupling channel 502B to set a resistance of the structure510B to the first resistance level indicative of the first digital valueor the second resistance level indicative of the second digital value,and output the second read current through the second spin-orbitalcoupling channel 502B to determine whether the resistance of thestructure 510B is at the first resistance level or the second resistancelevel, without outputting a current through the second magnet. As oneexample, the first magnet and first spin-orbital coupling channel may bearranged in a crossbar configuration with the second magnet and secondspin-orbital coupling channel. As another example, the first magnet andfirst spin-orbital coupling channel are arranged on top of the secondmagnet and second spin-orbital coupling channel from a 3D memoryarchitecture.

FIGS. 6A-6D are conceptual diagrams illustrating examples of memory celllayouts. Each memory cell of memory cells 601A-601D (collectively,memory cells 601) includes a bottom contact line 622A and a top contactline 622B. In some examples, a memory cell includes a third contact line622C, also referred to as a via. In some examples, each memory cellincludes a spin-orbital coupling channel 602, magnet 604, and selector606.

FIG. 6A illustrates an example memory cell 601A where the spin-orbitalcoupling channel 602 is coupled to the bottom contact line 622A. Themagnet 604 is coupled to the spin-orbital coupling channel 602 atinterface 603. A via 622C is also coupled to the spin-orbital couplingchannel 602. A selector 606 is coupled to, or is located within, the via622C, and couples to the top contact line 622B.

FIG. 6B illustrates an example memory cell 601B where the spin-orbitalcoupling channel 602 is coupled to the bottom contact line 622A. Themagnet 604 is coupled to the spin-orbital coupling channel 602 atinterface 603, and the selector line 606 is coupled to the spin-orbitalcoupling channel 602. A via 622C is coupled to the selector line 606 andthe top contact line 622B.

FIG. 6C illustrates an example memory cell 601C where the selector line606 is coupled to the bottom contact line 622A, and the spin-orbitalcoupling channel 602 is coupled to the top of the selector line 606. Themagnet 604 is coupled to the top of the spin-orbital coupling channel602 at interface 603. A via 622C is coupled to the spin-orbital couplingchannel 602 and to the top contact line 622A.

FIG. 6D illustrates an example of a vertical memory cell 601D. Aselector line 606 is coupled to the bottom contact line 622A. The magnet604 and spin-orbital coupling channel 602 are arranged verticallyrelative to and on top of the selector line 606. The magnet 604 andspin-orbital coupling channel 602 are coupled to one another atinterface 603. A top contact line 622B is coupled to the magnet 604 andspin-orbital coupling channel 602. Selector 606 may be directly coupledto bottom contact line 622A, spin-orbital coupling channel 602, andmagnet 604. Spin-orbital coupling channel 602 may be directly coupled tomagnet 604 and top contact line 622B. Similarly, magnet 604 may also bedirectly coupled to top contact line 622B. In some examples,spin-orbital coupling channel 602 and magnet 604 are aligned vertically,such that an interface 603 between the spin-orbital coupling channel 602and magnet 604 defines a plane that is substantially perpendicular to aplane defined by the interface between the magnet 604 and bottom contactline 622A and the interface between the spin-orbital channel 602 and thebottom contact line 622A.

The examples illustrated in FIGS. 5A-5C and FIGS. 6A-6D are provided asa few examples and should not be considered limiting. There may be otherconfigurations using USMR effects for constructing memory or logic cellsand the techniques are not limited to the examples provided in thisdisclosure.

FIGS. 7A and 7B are conceptual diagrams illustrating examples of crossbar memory architecture. FIG. 7A illustrates an example of a cross barmemory device 700A that includes a plurality of vertical memory cells701AA-701CC (collectively, memory cells 701) such as those illustratedin FIG. 6D. While cross bar memory device 700A includes three topcontact lines and three bottom contact lines with a total of nine memorycells 701, cross bar memory device 700A may include any number oftop/bottom contact lines 722 and memory cells 701.

FIG. 7B illustrates an example of a cross bar memory device 700B thatincludes a plurality of vertical memory cells 701AAA-701CCC(collectively, memory cells 701) in a 3D architecture. For instance,there may be stacks of cross bars on top of one another. While 3D crossbar memory device 700B includes three levels of memory cells stacked ontop of one another (with each level including three top contact linesand three bottom contact lines with a total of nine memory cells 701 perlevel), cross bar memory device 700B may include any number levels,where each level may include any number of top/bottom contact lines 722and memory cells 701.

In the examples of FIGS. 7A and 7B, each memory cell of memory cells 701includes a selector directly coupled to a bottom contact line, aspin-orbital coupling channel, and a magnet. Each spin-orbital couplingchannel may be directly coupled to a respective magnet and top contactline. Similarly, each magnet may be directly coupled to the respectivetop contact line. As illustrated in FIGS. 7A and 7B, each interfacebetween a respective spin-orbital coupling channel and magnet defines aplane that is substantially perpendicular to a plane defined by aninterface between the magnet and bottom contact line and the interfacebetween the spin-orbital channel and the bottom contact line.

Although FIGS. 7A and 7B are illustrated with vertical cells like thoseillustrated in FIG. 6D, the techniques are not so limited. In general,the various example memory cells described in this disclosure may beuseable for cross bar architecture, and the example of using thevertical cell is provided as one example to assist with understanding.

FIG. 8A is a conceptual diagram illustrating an example of nanowirecell. In some examples, a memory cell may be formed by nanowire of spinHall/magnetic material shelled in magnetic/spin Hall material. In thisexample, magnetization is then left/right hand circular representinginformation stored (e.g., left hand circular represents a first digitalvalue, and right hand circular represents a second digital value). Forexample, FIG. 8A illustrates a nanowire memory cell 850A that includes aspin-orbital coupling channel 802 surrounded by magnet 804 and ananowire memory cell 850B that includes a spin-orbital coupling channel802 surrounding magnet 804. Circular magnetization configurations maystill provide high/low USMR resistance states since the relativedirections between local magnetic moments and spins may be eitherparallel or anti-parallel.

FIG. 8B is a conceptual diagram illustrating an example of nanowirememory cells in cross bar memory device 800. In some examples, cells maybe made vertical with nanowires to further improve lateral density. Asillustrated in FIG. 8B, memory device 800 includes a plurality ofnanowire cells 850A. However, memory device 800 may include nanowirememory cells 850B in addition to, or in place of, nanowire memory cells850A. While memory device 800 include nine memory cells in a singlelevel, memory device 800 may include any number of levels (e.g., a 3Darchitecture) with any number of memory cells per level. Thus, a memorydevice 800 may include a plurality of nanowire cells 850 that each actas a memory device to store digital values (e.g., digital high ordigital low).

FIGS. 9A an 9B are conceptual diagrams illustrating an example techniquefor sensing magnetic nano-particles. For instance, the exampletechniques described in this disclosure may be used to detect proximityof magnetic particles using USMR. When magnetic particles 904 are absent(e.g., not proximate to the spin-orbital coupling channel, asillustrated in FIG. 9A), the spin-orbital coupling channel 902 shows noUSMR (e.g., the resistance across the channel may be relatively low).When magnetic particles 904 make contact with the spin-orbital couplingchannel 902 (e.g., as illustrated in FIG. 9B), the magnetic/spin Hallinterface is formed and USMR is present (e.g., the resistance increasesto a first or a second value) indicating the magnetization directions ofthe magnetic particles 904.

FIG. 9C is a graph illustrating example threshold for sensing presenceof magnetic nano-particles. For example, when there are no magneticnanoparticles proximate to the spin-orbital coupling channel, theresistance is relatively low. Then, as nanoparticles start to get moreproximate (e.g., closer) to the spin-orbital coupling channel, theresistance increases, and when it passes a threshold, a controller maydetermine that magnetic nanoparticles are proximate to the spin-orbitalcoupling channel. As the particle gets closer and closer to thespin-orbital coupling channel, the resistance increases, and thenplateaus after the particle makes contact with the spin-orbital couplingchannel.

In this disclosure, the large spin orbit coupling in topologicalinsulators results in helical spin-textured Dirac surface states thatare attractive for topological spintronics. These states generate anefficient spin-orbit torque on proximal magnetic moments at roomtemperature. However, memory or logic spin devices based upon suchswitching may require a non-optimal three terminal geometry, with twoterminals for the ‘writing’ current and one for ‘reading’ the state ofthe device. An alternative two-terminal device geometry is now possibleby exploiting the recent discovery of a unidirectional spin Hallmagnetoresistance in heavy metal/ferromagnet bilayers and (e.g., at lowtemperature) in magnetically doped topological insulatorheterostructures.

This disclosure describes observation of unidirectional spin Hallmagnetoresistance in a technologically relevant device geometry thatcombines a topological insulator with a ferromagnetic metal (includingconventional ferromagnetic metal). The example devices show afigure-of-merit (magnetoresistance per current density per totalresistance) that is comparable to the highest reported values inall-metal Ta/Co bilayers.

The spin Hall effect (SHE) in non-magnetic (NM) heavy metals originatesin their strong spin-orbit coupling (SOC). When a charge current flowsthrough a NM heavy metal, the SHE yields a spin accumulation at theinterface with a proximal material. If the latter is a ferromagnetic(FM) layer, the spin accumulation at the interface can exchange angularmomentum with the magnetic moments and exert a spin-orbit torque (SOT).In certain configurations and at sufficiently high charge currentdensity, the magnetization in the FM can be switched. SOT switching isbelieved to be potentially faster and more efficient than spin transfertorque (STT) switching that is typically used in magnetic tunnelingjunction (MTJ) devices for memory and logic applications.

SOT switching devices consist of a current carrying channel with aproximal nanomagnet whose magnetization determines the memory or logicstate. Conventional SOT switching devices need two terminals for‘writing’ the state of the device and an additional terminal, usually anMTJ on top of the nanomagnet, for ‘reading’ the magnetization state ofthe device. Since the stable states of the nanomagnet are180-degree-opposite to each other, symmetry prevents the sensing of themagnetization state using a conventional two terminal magnetoresistance,such as anisotropic magnetoresistance (AMR) or spin Hallmagnetoresistance (SMR). The required presence of a third terminal forreading makes such SOT switching devices more difficult to fabricate andusually less appealing for memory and logic applications.

With the recent discovery of unidirectional spin Hall magnetoresistance(USMR) in NM/FM bilayers, such as Pt/Co and Ta/Co, the third terminal ofSOT switching devices is no longer necessary. USMR originates from theinteractions between the spins generated at the NM-FM interface by SOCof the NM and the conduction channels in the FM. The unique feature ofUSMR is its symmetry; it is sensitive to two opposite magnetizationstates. Therefore, this disclosure describes a two terminal SOTswitching device that relies on USMR: the nanomagnet is switched by acurrent through the NM channel, while the state of the magnetization ofthe nanomagnet is simply read out using the USMR.

While much of the mainstream activity in SOT devices has focused onheavy metals, such as Ta, Pt and W, recent research has begun to explorethe potential of 3D topological insulators (TIs). These are narrow bandgap semiconductors wherein strong SOC and time-reversal symmetry yieldhelical spin-textured Dirac surface states whose spin and momentum areorthogonal. This ‘spin-momentum locking’ (SML) has been confirmed usingdirect measurements such as photoemission, electrical transport, andspin torque ferromagnetic resonance, as well as indirect means such asspin pumping. It has also been demonstrated that the spins can exerttorques on a FM as one would expect of SOT in the NM/FM case.

In comparison to the NM/FM bilayers, where SOT switching and sensingusing USMR have both been confirmed, the observation of USMR in TI/FMsystems is just beginning to emerge. A large USMR was observed inCr_(x)(Bi_(1-y)Sb_(y))_(2-x)Te₃/(Bi_(1-y)Sb_(y))₂Te₃ bilayer structuresat very low temperatures. Here, the Cr-doped layer is a FM TI with a lowCurie temperature and the other layer is a NM TI. For more pragmaticapplications, it is desirable to explore the USMR phenomenon isheterostructures that interface a TI with a conventional FM oftechnological relevance. Here, the experimental observation of USMR inTI/FM heterostructures, include (Bi,Sb)₂Te₃/CoFeB and Bi₂Se₃/CoFeBbilayers. As illustrated in FIGS. 3A and 3B, spins are generated due tothe SML of the TI when a charge current, j, is applied in the bilayer.Depending on the relative directions between the spins and magnetizationof FM, spins at the interface present different conductance wheninteracting with the conduction channels in the FM. The USMR in TI/FMsystems is similar to that in NM/FM systems with the differentmechanisms of spin generation.

In experiments described in this disclosure, USMR was observed attemperatures between approximately 20 K and approximately 150 K for(Bi,Sb)₂Te₃ (BST) and Bi₂Se₃ (BS). The largest USMR among the experimentsamples is about 2.7 times as large as the best USMR in Ta/Co sample, interms of USMR per total resistance per current density, observed in 6 QLBS and CoFeB of 5 nm bilayer.

The devices studied are fabricated from BST (t QL)/CoFeB (5)/MgO (2) andBS (t QL)/CoFeB (5)/MgO (2) thin film stacks (t=6 and 10), grown bymolecular beam epitaxy (MBE) and magnetron sputtering. Hall bars ofnominal length 50 μm and width 20 μm are tested with harmonicmeasurements under both longitudinal and transverse resistance setup.The magnetization of CoFeB is spontaneously in-plane with littleperpendicular anisotropy field.

FIG. 10A is a conceptual diagram illustrating resistance measurementsetup and definitions of rotation planes. FIG. 10A shows the definitionof the coordinates and rotation planes. Zero angles are at x+, z+ and z+directions for xy 10, zx 14 and zy 12 rotations respectively. Thedirections for rotation for increasing angle are indicated by arrows. A3 Tesla external field is applied and rotated in the xy, zx and zydevice planes while the first order resistance R_(ω) and second orderresistance R_(2ω) are recorded with 2 mA R.M.S. AC current.

FIGS. 10B and 10C are graphs illustrating resistance as a function ofangle. For example, FIGS. 10B and 10C show the angle dependencies ofR_(ω) and R_(2ω), respectively, of the BST (10 QL)/CoFeB (5 nm)/MgO (2nm) sample at 150 K. The R_(ω) exhibits typical SMR-like behavior withthe R^(x)>R^(z)>R^(y). Similar to the behavior seen in all metallicNM/FM bilayers, the variation of the second order resistance R_(2ω) withangle is also proportional to the magnetization projected along they-direction. The period of xy and zy rotations are 360 degrees while aflat line is observed in the zx rotation. The amplitude of R_(2ω) isabout 3 mΩ with an average current density of 0.667 MA/cm².

Due to Joule heating of the device and the temperature gradient acrossthe device plane, the anomalous Nernst effect (ANE) and spin Seebeckeffect (SSE) also contribute to the second order resistance. Tocarefully separate this contribution (denoted as R_(2ω) ^(ΔT)), from theUSMR, a series of measurements were carried out of Hall or transversesecond order resistance with xy-plane rotations under various externalfield strengths.

FIG. 11A is a conceptual diagram illustrating transverse/Hall resistancemeasurement setup. The transverse resistance is measured while theexternal field is rotated in the xy-plane. The second order Hallresistance, R_(2ω) ^(H), contains contributions from ANE/SSE, field-like(FL) SOT and anti-damping (AD) SOT. The ANE/SSE and AD SOT areproportional to cos φ while the FL SOT is proportional to cos 3φ+cos φ(ref²⁹).

FIGS. 11B and 11C are graphs illustrating resistance as a function ofangle and external fields, respectively. FIG. 11B shows two examples ofR_(2ω) ^(ΔT) vs. angle with 20 mT and 3 T external fields, respectively.Since AD SOT and FL SOT perturb the magnetization and thus contribute toR_(2ω) ^(H), their effects diminish at larger external field. FIG. 11Bshows that the data measured in a 20 mT field contain both cos φ and cos3φ components, while in a 3 T field, the data exhibit almost no cos 3φcomponent. There are two steps to obtain the R_(2ω) ^(ΔT). First, byfitting the angle dependent dat, the amplitudes of the cos φ and cos 3φcomponents can be extracted. The FL SOT can then be easily determinedand separated. This leaves the ANE/SSE and AD SOT. A plot of the datacorresponding to these contributions versus the reciprocal of totalfield is shown in FIG. 11C. In FIG. 11C, B_(dem)−B_(ani) is thedemagnetization field minus the perpendicular anisotropic field of theFM layer, which is determined to be about 1.5 T by separatemeasurements. Since the effect of the AD SOT will diminish at infinitefield, the intercept of the fitted line is the contribution of ANE/SSEto the 2 ^(nd) order Hall resistance. Then, the contribution of ANE/SSEto the longitudinal resistance R_(2ω) can be obtained by scaling thatfrom the Hall resistance with the relative ratio of device length todevice width. Finally, the USMR is determined once the ANE/SSEcontribution is subtracted from the R_(2ω).

FIGS. 12A and 12B are graphs illustrating contribution for totalresistance for different examples of structures that exhibit USMReffects. FIGS. 12A and 12B show the R_(2ω), R_(2ω) ^(ΔT) and R_(USMR) ofBST and BS samples with 2 mA and 3 mA current, respectively, at varioustemperatures. Temperature affects the chemical potential and therelative contributions to transport from surface and bulk conduction. Asa result, even though the magnetization and resistivity of the CoFeBlayer vary little within the range of temperature in the experiments,the charge to spin conversion in TIs and the related USMR are bothstrongly temperature dependent. The BST/CoFeB sample gives the highestUSMR at 70 K while the R_(2ω) and R_(2ω) ^(ΔT) keep increasing withincreasing temperature up to 150 K. The USMR of BS/CoFeB may only beconfirmed within between 50 K and 70 K because of larger noise andmagnetic field dependent signal outside the temperature window. At 70 K,BST and BS samples show resistance R^(z) of 733 Ω and 489 Ω, and USMRper current density of 1.067 mΩ/MA/cm² and 0.633 mΩ/MA/cm²,respectively. The ratios of USMR per current density to total resistanceof the two samples are 1.45 ppm/(MA/cm²) and 1.30 ppm/(MA/cm²),respectively. These values are slightly better than the best resultobtained using Ta/Co bilayers (1.14 ppm/(MA/cm²) at room temperature)⁹.

FIGS. 13A and 13B are graphs illustrating USMR per current density pertotal resistance and sheet USMR per current density, respectively, as afunction of temperature. For example, FIGS. 13A and 13B show USMR percurrent density per total resistance (R_(USMR)/j/R) and sheet USMR percurrent density (ΔR_(USMR)/j) of all four samples as a function oftemperature. This provides a more meaningful figure-of-merit forcomparisons of USMR across different types of samples. These two valuesalso show very similar trends for all samples at various temperatures,except for the comparison between BST6 and BS10 at 70K, in which BST6 islower than BS10 in terms of R_(USMR)/j/R but higher in terms ofΔR_(USMR)/j. The swap of position is mostly due to the larger totalresistance of BS10 compared to BST6 while they show similar R_(USMR)/j.The largest R_(USMR)/j/R and ΔR_(USMR)/j are 3.19 ppm/MA·cm² and 0.95mΩ/MA·cm², respectively, and both observed in BS6 at 150K. It is morethan as twice large as the best reported Ta/Co case. As mentionedbefore, the USMR measurements beyond the temperature ranges of the plotsof each sample show strong noise and field-dependent signal backgroundas to render the estimations of USMR unreliable.

The above demonstrates the presence of USMR in topologicalinsulator/ferromagnetic layer heterostructures. The USMR was observablewith a much lower current density compared to all metallic NM/FMbilayers. The ratios of USMR per current density to total resistance arefound to be comparable to the best result reported so far in Ta/Cobilayers. The observation of USMR in a TI/FM system is usable to build atwo terminal TI-based SOT switching device. Such a two-terminaltopological spintronic switching device is potentially more efficientcompared to MTJs that use STT switching due to the large SOC of TIs. Theobserved USMR may enable the read operation of such a device withouthaving to build a MTJ structure on top of TI. Such two terminal devicesmay be more architecture friendly and easier embed in current STTmagnetic random access memory architectures.

The following describes method for forming the bilayer structures toachieve the USMR effects. The Bi₂Se₃ or (Bi_(1-x)Sb_(x))₂Te₃ films weregrown by MBE on InP (111)A substrates. The InP (111)A substrate isinitially desorbed at 450° C. in an EPI (Veeco) 930 MBE under highpurity (7N) A_(s4) supplied by a Knudsen cell until a 2×2 reconstructionis visible in reflection high energy electron diffraction. The substrateis then moved under vacuum to an EPI 620 MBE for the Bi-chalcogenidedeposition. Bi₂Se₃ films were grown from high purity (5N) Bi and Seevaporated from Knudsen cells at a beam equivalent pressure flux ratioof 1:14. The substrate temperature was 325° C. (pyrometer reading of250° C.) and the growth rate was 0.17 nm/min. The films have a root meansquared (RMS) roughness of approximately 0.7 nm over a 25 μm² areameasured by atomic force microscopy (AFM). For (Bi,Sb)₂Te₃ films, theflux ratio of Bi to Sb was 1:3 and (Bi+Sb):Te is at a flux ratio ofapproximately 1:12 for a growth rate of 0.44 nm/min with a RMS roughnessof approximately 1.1 nm over a 25 μm² area measured by AFM. These filmsare grown at a substrate temperature of 315° C. (240° C. measured by apyrometer) using 5N purity Sb and 6N Te from Knudsen cells. Filmthickness is measured by X-ray reflectivity and crystal quality byhigh-resolution X-ray diffraction rocking curves of the (006) crystalplane—with a full width half max (FWHM) of approximately 0.28 and 0.11degrees for Bi₂Se₃ and (Bi,Sb)₂Te₃ films respectively.

The MBE-grown TIs were then sealed in Argon gas and transported to anultra-high vacuum (UHV) six-target Shamrock sputtering system whichcould achieve a based pressure better than 5×10⁻⁸ Torr at roomtemperature. The thin films were first gently etched by Argon ionmilling. Then the CoFeB layer was deposited using a Co₂₀Fe₆₀B₂₀ target.An MgO layer was deposited to serve as a protection layer. The devicefabrication began with a photolithography followed by an ion millingetching to define the Hall bars. Then the second photolithography and ane-beam evaporation followed by a liftoff were performed to makecontacts.

The devices were tested in a Quantum Design PPMS which providestemperature control, external field and rotation. The AC current at 10Hz was supplied by a Keithley 6221 current source. A Stanford ResearchSR830 or an EG&G 7265 lock-in amplifier paired with an EG&G 7260 lock-inamplifier were used to measure the first and second harmonic voltages,respectively and simultaneously.

FIG. 14 is a graph illustrating resistance as a function of magneticfield for different structures. For instance, FIG. 14 illustrates USMRin heavy metal/FM systems such as Ta/Fe and Pt/Fe.

FIGS. 15A-15C are graphs illustrating resistance as a function ofmagnetic field for additional structures are different temperatures.FIGS. 15A-15C illustrates the use of topological insulators asspin-orbital coupling channel. The surface states of a topologicalinsulator (TI) exhibit spin-moment locking. If a charge current isapplied across a TI channel, the electrons on top/bottom surface arealso spin-polarized to right/left. Although the physical process may bedifferent from spin Hall effect, the phenomena may be essentiallyequivalent (e.g., a change current in the channel induces spins ontop/bottom surface pointing right/left). Torques exerted on a FM by Tiswith planar Hall effect measurements have been observed.

FIGS. 15A-15C illustrate the USMR in TI/FM systems for different typesof bilayer structures. For example, FIG. 15A illustrates with bilayerstructure of (Bi,Sb)₂Te₃/CoFeB, and FIG. 15B illustrates with bilayerstructure of Bi₂Se₃/CoFeB. There may be even larger USMR when the CoFeBis replaced with magnetic insulator (MI), yttrium iron garnet (YIG), asillustrated in FIG. 15C.

FIG. 16 is a graph illustrating sheet USMR comparison. As illustrated inFIG. 16, the USMR of YIG/BS, in terms of sheet resistance per currentdensity, is an order of magnitude larger than the best USMR observedamong TI/CFB systems. The USMR may be as large as 50× of that in theTa(3)/Fe(2.5). Accordingly, USMR may provide potential of being furtherimproved towards practical applications. In some examples, the TIs servethe same role as other heavy metals or spin Hall materials. Thesematerial, including Tis, are referred to, generally, as spin Hallmaterial/channel and their functions of generating spins as spin Halleffect in general.

FIG. 17 is a flowchart illustrating example operations of a deviceconfigured to write to and read from a two-terminal spintronic device.For purposes of illustration only, the method of FIG. 17 will beexplained with reference to the example system 400 described in FIGS. 4;however, the method may apply to other examples.

Controller circuit 420 outputs a write current through a spin-orbitalcoupling channel 402 (1702). For example, controller circuit 420 mayoutput a pulse that has current density that satisfies (e.g., is greaterthan) a first threshold current density through contact 422A. The pulsemay generate a spin current by causing the spin of electrons 414 at theinterface of contact 422A and spin-orbital coupling channel 402 to alignin a particular direction. The spin current at the interface of contact422A and spin-orbital coupling channel 402 may cause the resistancestructure 410 to change from a first resistance level (e.g., indicativeof a digital high) to a second resistance level (e.g., indicative of adigital low). For example, the spin current may set the magnetizationdirection of magnet 404 from a first magnetization direction (e.g.,parallel) to a second magnetization direction (e.g., anti-parallel),which may change the resistance of interface 403, and thus changing theresistance of structure 410.

Controller circuit 420 outputs a read current through the spin-orbitalcoupling channel 402 (1704). For example, controller circuit 420 mayoutput a pulse that has a current density that does not satisfy (e.g.,is less than) a second threshold current density. The second thresholdcurrent density may be the same or different than the first thresholdcurrent density. Controller circuit 420 determines whether theresistance of structure 410 is at a first resistance value or a secondresistance value (1706). For example, controller circuit 420 maydetermine a voltage across structure 410 while outputting the readcurrent. The voltage across structure 410 may be indicative of theresistance of structure 410 (and hence the resistance of interface 403between spin-orbital coupling channel 402 and magnet 404). For example,when controller circuit 420 determines that the voltage across structure410 is a first voltage, controller circuit 420 may determine that theresistance of structure 410 is a first resistance value. Similarly, whencontroller circuit 420 determines that the voltage across structure 410is a second voltage, controller circuit 420 may determine that theresistance of structure 410 a second resistance value.

Responsive to determining that the resistance of structure 410 is at thefirst resistance value (“First” branch of 1706), controller circuit 420determines that the memory cell corresponds to a first digital value(1708). The first resistance value corresponds to a first digital value(e.g., one of a digital high or a digital low). Thus, in some examples,when controller circuit 420 determines that the resistance value ofstructure 410 corresponds to a first voltage, controller circuit 420 maydetermine that memory cell 401 represents a first digital value (e.g.,one of a “0” or “1”).

Responsive to determining that the resistance of structure 410 is at thesecond resistance value (“Second” branch of 1706), controller circuit420 determines that the memory cell corresponds to a second digitalvalue (1710). The second resistance value corresponds to a seconddigital value (e.g., the other of the digital high or the digital low).Thus, in some examples, when controller circuit 420 determines that theresistance value of structure 410 corresponds to a second voltage,controller circuit 420 may determine that memory cell 401 represents asecond digital value (e.g., the other of the “0” or “1”).

Various embodiments of the invention have been described. These andother embodiments are within the scope of the following claims.

What is claimed is:
 1. A device comprising: a first contact line; asecond contact line; a spin-orbital coupling channel coupled to, andpositioned between, the first contact line and second contact line; amagnet coupled to the spin-orbital coupling channel and positionedbetween the first contact line and the second contact line, wherein aresistance of the magnet and spin-orbital coupling channel is aunidirectional magnetoresistance; and wherein the magnet is directlycoupled to the spin-orbital coupling channel and at least one of thefirst contact line or the second contact line, wherein the spin-orbitalcoupling channel is directly coupled to the at least one of the firstcontact line or the second contact line, and wherein the unidirectionalresistance is not based on the magnetization direction of the magnetrelative to a magnetization direction of another magnet.
 2. The deviceof claim 1, wherein the unidirectional magnetoresistance of the magnetand spin-orbital coupling channel comprises a first resistance when amagnetization direction of the magnet is a first direction and a secondresistance when the magnetization direction of the magnet is a seconddirection.
 3. The device of claim 2, wherein the first magnetizationdirection is approximately 0 degrees and the second magnetizationdirection is approximately 180 degrees.
 4. The device of claim 1,wherein the magnet and spin-orbital coupling channel are arrangedvertically relative to one another.
 5. The device of claim 1, comprisinga first interface between the spin-orbital coupling channel and themagnet, wherein the first interface defines a first plane, wherein thefirst plane is substantially perpendicular to a second plane defined bya second interface between the magnet and the first contact line or athird plane defined by a third interface between the spin-orbitalcoupling channel and the first contact line.
 6. The device of claim 1,wherein the magnet is directly coupled both the first contact line andthe second contact line, and wherein the spin-orbital coupling channelis directly coupled to both the first contact line and the secondcontact line.
 7. The device of claim 1, further comprising a selector,wherein the selector is directly coupled to the magnet, the spin-orbitalchannel, and at least one of the first contact line or the secondcontact line.
 8. The device of claim 1, further comprising: a viacoupled to the first contact line; and a selector line coupled to thespin-orbital coupling channel and the via.
 9. The device of claim 1,wherein the spin-orbital coupling channel is formed from one or more of:a heavy metal, or a topological insulator.
 10. The device of claim 1,wherein the spin-orbital coupling channel comprises a first spin-orbitalcoupling channel, the magnet comprises a first magnet, the devicefurther comprising: a first interface between the first spin-orbitalcoupling channel and the first magnet; a third contact line; a secondspin-orbital coupling channel coupled to, and positioned between, thesecond contact line and the third contact line; and a second a magnetcoupled to the second spin-orbital coupling channel and positionedbetween the second contact line and the third contact line, wherein aresistance of the second magnet and second spin-orbital coupling channelis a unidirectional magnetoresistance.
 11. The device of claim 1,wherein the spin-orbital coupling channel comprises a first spin-orbitalcoupling channel and the magnet comprises a first magnet, the devicefurther comprising: a plurality of spin-orbital coupling channels andcorresponding plurality of magnets sandwiched between the first contactline and the second contact line, wherein a resistance of a respectivespin-orbital coupling channel of the plurality of spin-orbital couplingchannels and the corresponding magnet of the plurality of magnets is aunidirectional magnetoresistance.
 12. The device of claim 1, furthercomprising a controller circuit configured to: output a write currentthrough the spin-orbital coupling channel to set a resistance of themagnet to a first resistance level indicative of a first digital valueor a second resistance level indicative of a second digital value; andoutput the read current through the spin-orbital coupling channel todetermine whether the unidirectional magnetoresistance is at the firstresistance level or the second resistance level, without outputting acurrent through the magnet.
 13. A device comprising: a first contactline; a second contact line; a third contact line; a first spin-orbitalcoupling channel positioned between the first contact line and secondcontact line and directly coupled to at least one of the first contactline or the second contact line; a second spin-orbital coupling channelpositioned between the second contact line and the third contact lineand directly coupled to at least one of the second contact line or thethird contact line; a first magnet positioned between the first contactline and the second contact line and directly coupled to the firstspin-orbital coupling channel, wherein a resistance of the magnet andspin-orbital coupling channel is a first unidirectionalmagnetoresistance; and a second magnet positioned between the secondcontact line and the third contact line and directly coupled to thesecond spin-orbital coupling channel, wherein a resistance of the secondmagnet and second spin-orbital coupling channel is a secondunidirectional magnetoresistance.
 14. The device of claim 13, whereinthe first magnet is directly coupled to the at least one of the firstcontact line or the second contact line, and wherein the second magnetis directly coupled to the at least one of the second contact line orthe third contact line.
 15. The device of claim 13, further comprising:a first selector and a second selector, wherein the first selector isdirectly coupled to the first magnet, the first spin-orbital channel,and at least one of the first contact line or the second contact line,and wherein the second selector is directly coupled to the secondmagnet, the second spin-orbital channel, and at least one of the secondcontact line or the third contact line.
 16. The device of claim 13,wherein the first magnet and first spin-orbital coupling channel arearranged in a crossbar configuration with the second magnet and secondspin-orbital coupling channel.
 17. The device of claim 13, wherein thefirst magnet and first spin-orbital coupling channel are arranged on topof the second magnet and second spin-orbital coupling channel to form a3D memory architecture.
 18. A device comprising: a first contact line; asecond contact line; a spin-orbital coupling channel configured toreceive a read current and a write current, wherein the spin-orbitalcoupling channel is coupled to, and is positioned between, the firstcontact line and second contact line; a magnet coupled to thespin-orbital coupling channel and positioned between the first contactline and the second contact line, wherein a resistance of the magnet andspin-orbital coupling channel is a unidirectional magnetoresistance; anda controller circuit configured to: output a write current through thespin-orbital coupling channel to set a resistance of the magnet to afirst resistance level indicative of a first digital value or a secondresistance level indicative of a second digital value; and output theread current through the spin-orbital coupling channel to determinewhether the unidirectional magnetoresistance is at the first resistancelevel or the second resistance level, without outputting a currentthrough the magnet.
 19. The device of claim 18, wherein the writecurrent comprises a first current density greater than or equal to athreshold current density for setting a magnetization direction of themagnet, and wherein the read current comprises a second current densityless than the threshold current density.
 20. The device of claim 18,wherein the magnet is directly coupled to the spin-orbital couplingchannel and at least one of the first contact line or the second contactline, and, and wherein the spin-orbital coupling channel is directlycoupled to the at least one of the first contact line or the secondcontact line.
 21. The device of claim 18, wherein the spin-orbitalcoupling channel comprises a first spin-orbital coupling channel, themagnet comprises a first magnet, the read current comprises a first readcurrent, and the resistance of the first magnet and the firstspin-orbital coupling channel is a first resistance, the device furthercomprising: a third contact line; a second spin-orbital coupling channelconfigured to receive a second read current, wherein the secondspin-orbital coupling channel is coupled to, and is positioned between,the second contact line and the third contact line; and a second amagnet coupled to the second spin-orbital coupling channel andpositioned between the second contact line and the third contact line,wherein a resistance of the second magnet and second spin-orbitalcoupling channel is a second unidirectional magnetoresistance, whereinthe controller circuit is configured to: output the second read currentthrough the second spin-orbital coupling channel to determine whetherthe resistance of the second interface is at the first resistance levelor the second resistance level, without outputting a current through thesecond magnet